As one of high-density recording media, there is known a magneto-optical recording medium (hereinafter referred to as a magneto-optical disc) which performs recording and reproduction of information by applying an external magnetic field simultaneously with irradiation of laser light.
In recent years, further increase in recording capacity of the magneto-optical disc has been desired. To increase the recording capacity of the disc, the size of recording bits and the bit pitch in circumferential direction of the disc needs to be further reduced.
In general, the size of recording bits is determined by the diameter of the emitted laser spot. However, there has been proposed magnetically induced super resolution (MSR) reproduction method which allows for the reproduction of bits having a pitch smaller than the diameter of the spot (Japanese Unexamined Patent Publication No. 2000-200448). A magneto-optical recording medium which can be reproduced by the MSR reproduction method has a recording section of a three-layer structure including a reproducing layer, an intermediate layer, and a recording layer.
The MSR reproduction method is also referred to as a double mask RAD (Rear Aperture Detection) reproduction method because it uses a low-temperature area and a high-temperature area in a laser spot as magnetic mask regions and transfers, within a mid-temperature, the bits recorded in the recording layer to the reproducing layer area to read out the bits.
The principle of the MSR reproduction method will be described below.
In FIG. 16, a sectional view illustrating the constitution of a conventional magneto-optical disc is shown. In a recording layer 105, bits of size smaller than the diameter of the laser spot are recorded therein.
First, just before reproducing laser light is emitted, an initializing magnet is used to apply an initializing magnetic field to the disc, so that the magnetizations of a reproducing layer 103 and an intermediate layer 104 are directed to the same direction as the initializing magnetic field.
When the low-temperature area of the disc is irradiated with the reproducing laser light at the reproduction of the bits, the magnetizations of the reproducing layer 103 cover the bits recorded in the recording layer 105 to form a so-called front mask.
In the high-temperature area of the disc within the light beam spot where its temperature exceeds the Curie temperature of the intermediate layer 104, the exchange coupling exerted between the recording layer 105 and the reproducing layer 103 is broken and the magnetizations of the reproducing layer 103 in the high-temperature area are directed to the direction of the reproducing magnetic field applied from the outside. Consequently, in the high-temperature area, the reproducing layer 103 serves as a mask (rear mask) to cover the bits.
In an area (mid-temperature area) between the high-temperature and low-temperature areas serving as the masks in the portion of the disc irradiated with the light beam, the bits recorded in the recording layer 105 are transferred to the reproducing layer 103 through the intermediate layer 104. The transferred bits are irradiated with the reproducing laser light and the reflected light of the reproducing laser light is received by a light detector. Then, a Kerr rotation angle is detected so that recording bits recorded in the recording layer are reproduced.
With such a conventional MSR reproduction method, the bits can be reproduced from the mid-temperature area of a size smaller than the spot diameter of the light beam, thereby allowing for high resolution.
In the conventional MSR reproduction method, laser light having a wavelength of about 600 to 700 nm is generally used. If a laser beam having a shorter wavelength can be used, the minimum spot diameter that can be diffracted is made smaller, whereby the size of the recording bit can be reduced. Accordingly, the recording capacity of the magneto-optical disc of MSR reproduction type can be increased.
However, a GdFeCo film adopted as the reproducing layer 103 of the conventional magneto-optical disc of MSR reproduction type gives, when irradiated with a light beam having a short wavelength in the range of 350 nm to 450 nm, only a small Kerr rotation angle. Therefore, a magneto-optical effect sufficient for the reproduction can not be achieved.
Further, with the use of the shortwave laser light, a problem arises that a photodetector sensitivity decreases, and thus a reproduction signal (carrier level) is weakened.